[en] The goal of the Thermionic Fuel Element Verification Program (TFEVP) is to demonstrate the technological readiness of a thermionic fuel element in a thermionic reactor having an electric power output in the 0.5- to 5-MW(electric) range and a full-power life of 7 yr. The TFEVP has made significant progress in developing components capable of withstanding the required neutron fluence (4 x 10²² n/cm², E > 0. 1 MeV) and the required burnup (5.3%) of a 2-MW(electric) system. Technology developed under the TFEVP also supports the 5- to 40-kW(electric) thermionic systems currently of interest to the Strategic Defense Initiative Organization and the US Air Force. The fast-neutron flux in certain 5- to 40-kW(electric) systems is up to a factor of 7 less than that in 0.5- to 5-MW(electric) systems. Component technology that has been developed for 0.5- to 5-MW(electric) systems will thus be suitable for use in long-life, high-performance, 5- to 40-kW(electric) systems. Components that are being developed by the TFEVP include insulator seals, sheath insulators, fueled emitters, cesium reservoirs, and inter- connective TFE components. In addition, the TFEVP has created a preliminary 2-MW(electric) system design and is currently evaluating converter performance under various conditions. Prototypical TFEs are also being tested. The TFEVP is developing accurate converter-performance models that are correlated to observed test data